Photoexcitation of perovskite precursor solution to induce high-valent iodoplumbate species for wide bandgap perovskite solar cells with enhanced photocurrent

Solution-processed organic–inorganic hybrid perovskite solar cells are among the candidates to replace the traditional silicon solar cells due to their excellent power conversion efficiency (PCE). Despite this considerable progress, understanding the properties of the perovskite precursor solution is critical for perovskite solar cells (PSCs) to achieve high performance and reproducibility. However, the exploration of perovskite precursor chemistry and its effects on photovoltaic performances has been limited thus far. Herein, we modified the equilibrium of chemical species inside the precursor solution using different photoenergy and heat pathways to identify the corresponding perovskite film formation. The illuminated perovskite precursors exhibited a higher density of high-valent iodoplumbate species, resulting in the fabricated perovskite films with reduced defect density and uniform distribution. Conclusively, the perovskite solar cells prepared by the photoaged precursor solution had not only improved PCE but also enhanced current density, confirmed by device performance, conductive atomic force microscopy (C-AFM), and external quantum efficiency (EQE). This innovative precursor photoexcitation is a simple and effective physical process for boosting perovskite morphology and current density.

Hybrid organic-inorganic perovskite materials are innovative materials beneficial for countless applications due to minimal materials usage along with great practical impact. Within a short period of time, organic-inorganic lead halide perovskites have reached record certified power conversion efficiencies (PCEs) now exceeding 25.7% 1 unprecedented in the photovoltaics field. Perovskite materials have demonstrated powerful applications in solar cells and have gained tremendous attention for various applications in optoelectronics. The exceptional efficiency outputs of perovskite solar cells are due to their excellent materials properties 2 which include high optical absorption coefficient 3 , long-balanced charge carrier diffusion length [4][5][6][7] , low exciton binding energies, simple of band gaps tuning 7,8 via substitutions of the precursor components. Among the perovskite materials, FAPbI 3 -based perovskite exhibits high charge-carrier extraction and a broadening absorption into the near-infrared because of their band gap (1.48 eV) which is closer to the optimum value of a single-junction solar cell [9][10][11] . However, the stability can be an issue in these cells. FAPbI 3 -type structure commonly has two phase structures: a perovskite black α-phase and a non-perovskite yellow δ-phase. Only the α-phase perovskite is a suitable photoactive phase 9,12 while this phase readily transforms to the yellow δ-phase because of the large size of the FA cation. Therefore, inhibition of phase transformation can be accomplished by substituting some FA cation species with MA or Cs

Experimental section
Perovskite film preparation. The main triple cation perovskite formula Cs 0.05 FA 0.73 MA 0.22 Pb(I 0.77 Br 0.23 ) 3 was fabricated by the procedure described in Supplementary Information and the previous work 28 ; other types of perovskites such as MAPbI 3 , Cs 0.17 FA 0.83 PbI 2.49 Br 0.51 (CsFA), and Cs 0.05 FA 0.81 MA 0.14 PbI 2.55 Br 0.45 (CsFAMA) were also fabricated. (See materials and experimental details in Supplementary Information). The semi-transparent perovskite can be obtained by tuning the composition between I − and Br − ratio (confirmed by UV-Vis spectra and optical bandgap as shown in Fig. S1a, b). 1.5 M triple cation perovskite solutions were divided by four sets for each experiment. The solution was placed at room temperature (RT) for 30 min under N 2 environment in a glovebox as a control sample. Three other sets of solutions were placed under different excitations: (1) on hotplate at 60 • C for 30 min, (2) under UV light at 0.015 mW/cm 2 at 25 °C for 30 min, and (3) under 1 sun illumination at 100 mW/cm 2 at 25 °C for 30 min prior to the film deposition in N 2 environment. The samples were abbreviated as RT, 60 • C, UV, and 1-Sun. To study the dynamics after light excitation, excited perovskite solutions were investigated and spin casted after specific different time durations (0, 10, 30, 90, and 360 min). To study the phase stability and the evolution of I-and Br-rich regions in the wide bandgap triple cation perovskite layer, PL measurement of all perovskite films was conducted after different time durations (0, 5, 10, 30, and 60 min) under 1 sun illumination. We performed additional experiments to assess the stability of all perovskite devices under 1 sun illumination. To accelerate the degradation of the devices, we stored them in a humiditycontrolled dry box at a relative humidity of 40-60% and room temperature for 45 days without encapsulation. All experimental conditions were summarized in Fig. 2a.
Perovskite solar cell fabrication. The 2.5 cm by 2.5 cm FTO/SnO 2 substrates were prepared from SnCl 2 ·2H 2 O powder dissolved in ethanol at 0.2 M and kept for 2 days at room temperature before use. Then, the solution was deposited onto FTO glass via spin coating at 3000 rpm with initial acceleration of 1500 rpm/s for 30 s under ambient conditions and annealed at 180 • C for 1 h and cooled down under room temperature. This method was used in our previous publication 29 . Prior to perovskite deposition, FTO/SnO 2 substrates were treated in a UV ozone cleaner for surface cleaning. 50 µl of triple cation perovskite solution (Cs 0.05 FA 0.73 MA 0.22 Pb(I 0.77 Br 0.23 ) 3 ) was then spread on the substrate and spun using one-step spin coating process at 3500 rpm for 35 s with 700 rpm/s acceleration. 100 µl anisole was then dripped on the film at 30 s as the antisolvent after starting the program. The films were then annealed at 100 °C for 30 min. The whole spinning and www.nature.com/scientificreports/ annealing processes were done under a N 2 -filled glovebox. Spiro-OMeTAD as hole transport material (HTM) was prepared by dissolving 80 mg of spiro-OMeTAD in 1 ml of chlorobenzene; 28.5 µl of 4-tert-butylpyridine and 17.5 µl of Li-TFSI solution (520 mg in 1 ml acetonitrile) were added into the spiro-OMeTAD solution and stirred overnight at room temperature. The solution with the volume of 60 µ l was dropped and rested for 30 s on top of the perovskite layer before starting the spin-coating process with spin speed of 2000 rpm for 30 s at 1000 rpm/s acceleration. The deposited samples were kept in a glovebox for overnight. To make 80-μm-thick carbon electrode, commercial carbon ink was doctor bladed on a glass slide and soaked in ethanol for two hours; the carbon layer could then be peeled off, dried at room temperature, and cut for further usage 30,31 . Approximately 0.04 cm 2 of square carbon sheet was placed on top of HTM and then covered by ITO glass. Eventually, the whole stack was pressed at 0.6 MPa at 60 • C for 5 min to finish the full device.
Characterization methods. X-ray diffraction measurements were carried out by Bruker D8 Discover X-ray diffractometer (Cu anode material, detector scan mode using a step size of 0.01°, 0.4 s per step, and 2θ from 5° to 45°). Zetasizer Helix Particle Analyzer from Malvern Panalytical was employed to detect the size distribution. Surface morphologies and cross-sections were observed by scanning electron microscopy (SEM; JSM-7610FPlus JEOL, tungsten filament electron source, 20 kV, and secondary electron mode). The optical absorption spectra were obtained by using a UV-Vis spectrophotometer (Shimadzu UV-2600, 900-300 nm, medium mode, and absorbance mode). The Photoluminescence spectra were recorded by Horiba FluoroMax4 + spectrofluorometer (integration time of 0.1 s, excitation of 500 nm, excitation slit of 10 nm, emission wavelength measurement between 650 and 850 nm, and emission slit of 5 nm). For stability testing, films were kept in the dark and stored in a humidity control dry box. The relative humidity (RH) was fluctuated, ranging from 40% to 60%. 1 sun irradiation (100 mW/cm 2 ) was provided by AAA-class 7520-LED light source with LSS-7120 LED controller (VeraSol). 4 W LED 6500 K (Philipe, E27, cool daylight) was used as an indoor light source. The light intensity was calibrated by Si diode (Hamamatsu S1133). Solar We also observe another peak at 11.57 • of the non-perovskite yellow δ-phase, as this phase is more stable than the active black α-phase of FAPbI 3 at room temperature 34 . The narrow view of perovskite crystal planes measured between 10° and 16° is shown in Fig. S2b. For the main (110) peak at 14.21 • , there is no peak shifting observed for all conditions. Interestingly, the dominant diffraction peak intensity of perovskite films prepared from the solution with UV illumination is higher than to that of RT sample by 10%, indicating higher crystallinity and reduced phase impurity 27 . The peak intensity of 60 • C sample is similar to that of the UV sample. For 1-Sun sample, the sharp (110) diffraction peak at 14.21° is intensified by 30% as a result of more ordered crystal formation compared to that of RT. Narrow view of (200) and (202) perovskite crystal planes are shown in Fig. 1b. However, perovskite films prepared from UV and 1-Sun solutions display shifts towards lower diffraction angles at 20.12° (200) and 24.69° (202), respectively, referring to structural expansion in the unit cell parameters caused by the chemical species in a precursor solution change with illumination. We hypothesize that a peak shift originates from the transformation to the orthorhombic crystal structure with space group Amm2, corresponding to α-FASnI 3 perovskite phase 35 within the triple cation perovskite. The existence of the inactive perovskite phase (δ) can be emphasized by the ratio of active phase and inactive perovskite phase intensities measured at 14.21° and 11.57° (⍺-phase/δ-phase) as shown in Fig. 1c. For the 1-Sun sample, the ratio is much higher than those www.nature.com/scientificreports/ from other conditions, illustrating best film quality from the treatment. Figure 1d illustrates wider full-width at half maximum (FWHM) at (110) for 60 • C compared to that of the RT sample; the broadening is explained by a microscale structural inhomogeneity, 36 likely due to faster crystallization with elevated temperature. FWHMs are decreased to 0.114° and 0.110° from 0.116° for UV and 1-Sun films, respectively. Both values are smaller compared to that of the RT sample, implying improvement in crystallinity and/or more preferred (110) orientation with inactive perovskite phase suppression. The black line in Fig. 1d illustrates average crystallite size of each specific lattice plane according to the Scherrer equation. The lattice strains of perovskite films after different stimuli are estimated using the Williamson-Hall analysis. As shown in Fig. 1e, the lattice strains are 1.14 × 10 -3 and 1.22 × 10 -3 for RT and 60 °C, showing increased strain with treatment. However, the strain value decreases to 1.00 × 10 -3 upon UV light treatment. The lattice strain value of 1-Sun sample is between those of 60 °Cand UV-samples. Lower lattice strain typically links to high stability and charge transport 37 . Nishimura et al. reported that carrier mobility is related to lattice strain, which affects carrier collection 38 . Carrier extractions can be enhanced by the decreasing strains in Pb perovskite layer 39 . UV light soaking possibly changes the chemical environment at grain boundaries, therefore reducing lattice strain. By minimizing the strain on the lattice, the formation of defect centers or traps that may capture charge carriers and have an adverse impact on the efficiency of the solar cell can be reduced 40 .     27 ), leading to high-quality perovskite films with reduced internal disorder and less iodide vacancy defects. The mechanism is in good agreement with the previous reports 27, 43,44 , which demonstrate that high-valent iodoplumbate species cause high-quality perovskite films by decreasing donor defects such as iodide vacancies.

Energy stimuli and perovskite colloidal solutions.
Optical properties. To investigate the film optical properties, absorption and emission of different perovskite films are investigated. As shown in Fig. 3a, all perovskite samples with the same thickness of 620 nm (Fig. S1c) show quite similar absorption characteristic, indicating high quality perovskite films. The optical bandgap is calculated by Tauc plot based on direct bandgap property as shown in Fig. 3b. The bandgap of our control is 1.66 eV which is in agreement with previous report 28 . However, the bandgaps of perovskite films prepared from perovskite solutions with different energy stimuli are increased from 1.66 to 1.68 eV. Therefore, photo-excited solution pathway plays an important role in enlarging the bandgap which is in agreement with the structural expansion in the unit cell parameters seen in the previously-discussed XRD results. We determined the photoluminescence (PL) emission spectra to evaluate the charge transfer dynamics of perovskite films for all conditions. The films are measured via an excitation wavelength of 500 nm as shown in Fig. 3c. Each sample was deposited on a FTO/SnO 2 substrate. 1-Sun sample shows the strongest PL intensity, which can result from the suppressed non-radiative SRH recombination [45][46][47] . The PL intensities was normalized in order to assess shifting induced by different energy stimuli. The 1-Sun peak position is red-shifted by 5 nm compared to that of RT sample, confirming an increasing portion of ⍺-FAPbI 3 within the bulk perovskite layer 48 Fig. S6). The illuminated perovskite precursor solutions were tested by FTIR measurement as seen in Fig. 4b-d. The N-H stretch (around 3400 and 3250 cm -1 ), which represents FA + /MA + in perovskite precursor, can be identified after the idle time duration of 10 min and further increases and slightly shifts with longer time durations (see the FTIR spectra of FAI solution in Fig. S7). PL intensity of fresh and aged perovskite films are exhibited in Fig. 4e-f, respectively. The PL intensity of the 10 min idle duration is the strongest for both font and back sides of the film, indicating lower non-radiative recombination. The peak intensities decrease after more than 10 min of no illumination, signaling decreased spontaneous radiative recombination caused by trap states on the surface and/or grain boundaries of the perovskite layer. At the same time, we observe the double peak PL spectra of perovskite thin films, indicating different phases occurring along with increasing idle time durations.
Electrical properties. The effects of energy stimuli applied to perovskite solutions on current-morphology correlation at the nanoscale were revealed by conductive atomic force microscopy (C-AFM). During the measurement, the perovskite films were excited by a white LED source (0.2 mW/cm 2 ). Figure 5a-d show surface morphology and corresponding photocurrent mapping of the samples. We observe a slight increase in the rootmean-square surface roughness (RMS) from 23.6 nm (pristine sample) to 29.5 nm (60 °C sample). In contrast to photoexcited samples, RMS diminishes to 25.4 nm and dramatically drops to 19.4 nm for UV and 1-Sun, respectively. The reduction in RMS values and grain size could be due to more colloids, which form nucleation sites in those samples 27,43 . The open-circuit map (V oc map) is investigated on FTO/SnO 2 /perovskite stack by setting a forward bias of 0.6 V to the AFM cantilever to simulate V oc environment where the photocurrent is not far from zero as shown in Fig. 5e-h. To account for the higher traps due to the lack of the hole transport layer (HTL) in this experiment, the bias is assumed to be 0.6 V instead around 1.1 V in the actual solar device as shown in Table S1. The area with positive current in V oc map represents the region where V oc is more than 0.6 V, indicating perovskite surface with lower trap density 52 . Figure 5e-h show that the photocurrent decreases from 104.9 pA to 58.5 pA with the increase of the temperature from RT to 60 °C, linking to small grain and therefore more defects of the 60 °C treatment. The photocurrents are slightly reduced to 89.4 pA and 86.5 pA for UV and 1-Sun samples, agreeing with the grain size distribution trend (Fig. S8e) as observed in SEM (Fig. S8a-d). The slightly smaller grain sizes are caused by more nucleation sites stemmed from more [PbI 6 ] 4− colloids. The short circuit current map (I sc map) was done to evaluate charge conductivity pixel by pixel at zero bias under 0.2 mW/ cm 2 irradiation as shown in Fig. 5i-l. Interestingly, both UV and 1 sun illuminated samples exhibit significantly higher photocurrents compared to those of RT and 60 °C samples, indicating superior charge transport 53 on the grain surfaces due to less iodide vacancies from more [PbI 6 ] 4− population; however, lower photocurrents and LED (illuminance: 1000 lux, 0.31 mW/cm 2 ) to investigate the incident light-dependent photovoltaic performances for outdoor and indoor usages. The average photovoltaic parameters, which include V oc , J sc , FF, and PCE are summarized in Table S1. All devices were fabricated using low-cost carbon electrode. The schematic of n-i-p device configuration (FTO/SnO 2 /perovskite/spiro-OMeTAD/carbon/ITO) is shown in Fig. 6a. Besides, cross-sectional SEM images of the carbon-based architecture is shown in Fig. 6b, showing large grain, smooth, and dense layers for the 1-Sun sample. Figure 6c illustrates the carbon electrode in the device. Figure 6d shows J-V characteristics of the best devices from RT, 60 °C, UV, and 1-Sun conditions. The PCE of UV and 1-Sun devices are superior to those of RT and 60 °C conditions with the PCEs of 13.62% and 13.25%, respectively. The PCE performances under low light condition show a similar trend with 1 sun illumination. The J-V curves of the best devices under low light condition and an irradiance spectrum of indoor light source (cool daylight LED) are shown in Fig. S9a,b. Moreover, the highest and second highest current density (J sc ) of 18.2 mA/cm 2 and 17.5 mA/cm 2 are observed in UV and 1 sun treated devices, respectively. The clearly improved current density is due to reduced vacancy defects from more full-coordination iodoplumbates 27 . These results are in agreement with the relatively good photocurrents confirmed by C-AFM as shown in Fig. 5i-l. The ratio of generated electrons to given photons at a specific wavelength of light excitation were identified; the external quantum efficiency (EQE) spectra of the best PSC devices are displayed in Fig. 6e. The higher EQE results for 1-Sun and UV samples are consistent with the J sc from J-V performances and the photocurrent results in the I sc maps from C-AFM. We also applied our external stimuli methods to other popular types of perovskites which are MAPbI 3 , Cs 0. 17  The problem of wide-bandgap perovskite with high bromine is phase instability. The phase segregation between iodine-and bromide-rich regions in the perovskite layer under light illumination is observed through in PL measurement as shown in Fig. 7a. In fact, the iodine-rich region can lead to charge recombination, which can reduce the V oc and FF under AM1.5G irradiation 54,55 . The PL shift began within 5 min of 1 sun irradiation for all perovskite films. After 60 min of 1 sun illumination, the 1-Sun and UV films exhibited smaller photoluminescence peak shifts compared to those of control and 60 °C conditions, which suggest higher phase stability. We also investigated the stability of perovskite solar cells (PSCs), which is a crucial issue for commercialization. Figure 7b-e shows the normalized PCE of the unsealed PSCs under 1 sun illumination. After 45 days, the PCE decreased along with other parameters. However, the PSCs subjected to 1 sun and UV irradiation show smaller changes in PCE, indicating improved device stability due to the photoenergy pathway.
To demonstrate the potential as top cell material for tandem architecture due to its high current, the 1 suntreated precursor was used to make perovskite cells with transparent electrodes (ITO), which were fabricated by different processes, radio frequency (RF), direct current (DC), and Ar/O 2 direct current (Ar/O 2 DC) magnetron sputtering. The samples are labelled as RF, DC, and Ar/O 2 DC devices, respectively. The J-V characteristic was performed with an active area of 0.25 cm 2 ; the inset of Fig. 8a show cross-sectional SEM of the device architecture. As shown in Fig. 8a, different ITO electrodes mainly affect FF and J sc , while V oc remains comparable to that of the carbon electrode. The solar cell performances are shown in Table S2. The Ar/O 2 DC device shows lowest R s of 5.59 Ωcm 2 , which is caused by good contact between ITO and HTL, leading to the excellent FF value of 0.78 and the PCE of 17.9%. However, the poor R s 's of 10.42 Ωcm 2 from RF and 61.35 Ωcm 2 from DC are revealed, causing low FF and PCE. Furthermore, the performance of larger scale device from Ar/O 2 DC process with an active area of 1.00 cm 2 was also measured as shown in Fig. 8b. The large-scale device shows the PCE of 11.2% with J sc of 15.13 mA/cm 2 , V oc of 1.11 V, and FF of 0.67. The statistics are shown in Table S2.

Conclusions
In this work, we modified the iodoplumbate equilibrium in a triple cation perovskite precursor system to investigate the correlation between iodoplumbate species within the perovskite solutions under different external stimuli. 1 sun and UV irradiation can easily affect the equilibrium without any additives, leading to great conversion to high-valent iodoplumbate species in perovskite precursor solutions. For the illuminated perovskite precursors, the crystallinity is greatly enhanced along with smooth morphology and reduced defect density. The resulting films from UV and 1-Sun precursor solutions have improved photocurrents as seen from C-AFM and www.nature.com/scientificreports/ solar cell performances due to reduced donor defects such as iodide vacancies from high-valent iodoplumbate species. With 1 sun treatment, the PCEs of 13.6% and 17.9% (cell area of 0.25 cm 2 ) were obtained under 1 sun illumination by using low-cost carbon and ITO as the electrodes, respectively. The J sc is significantly better than those of the untreated and the thermal-treated perovskite solutions. Moreover, the same concept was further demonstrated with a large-scale semi-transparent device having transparent electrode and the cell area of 1.00 cm 2 , yielding the PCE of 11.2% along with the J sc of 15.13 mA/cm 2 , the V oc of 1.11 V, and the FF of 0.67. Our results suggest that precise control of chemical environment of iodoplumbates in perovskite precursor solution by light treatment is critical for fabricating highly efficient PSCs. While the process was mainly tested on a widebandgap material for silicon/perovskite tandem solar cell technology and low light photovoltaic, this similar approach is likely useful for other perovskite compositions for various optoelectronic applications.

Data availability
The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.